Handsets and other wireless communication devices are undergoing miniaturization. As a result, there is a continuing interest in small antennas that can be internally mounted to the handsets.
In addition, it is desirable for radiophones to be able to operate within multiple frequency bands in order to utilize more than one communication system. For example, the frequency bands allocated in North America are from 824 to 894 MHz for Advanced Mobile Phone Service (AMPS, or Cell band) and from 1850 to 1990 MHz for Personal Communication Services (PCS). GSM (Global System for Mobile communications) is a digital mobile telephone system operating between 880 MHz and 960 MHz. DCS (Digital Communications System) is a digital mobile telephone system operating between 1710 MHz and 1880 MHz. WCDMA (wideband code division multiple access) systems are now deployed over the world (operating frequencies for the WCDMA systems are 1920-2170 MHz for Europe/Asia and 1850-1990 MHz for the USA). To enable mobile handsets to work over service areas with different frequency bands, dual band, triple band, and quad band antennas are needed.
For the 3G systems such as WCDMA and CDMA2000, there are optional requirements specifically for diversity antennas operating at the same frequency band in hand held devices (e.g., in mobile phones, handsets, etc.). Isolation levels between the main antenna and the diversity antenna affect the overall performance of diversity antenna systems in terms of correlation coefficient and radiation efficiency. Therefore, isolation among antennas with different bands as well as in the same band is important for multi-radio terminals.
In ongoing applicant project, due to metallic front and back covers, the BLUETOOTH/WLAN antenna (with operating frequencies 2400-2484 MHz) has been forced to be placed close to the main GSM antenna. Consequently, the isolation between the main and the BLUETOOTH/WLAN antenna is around 12 dB, being 3 dB less than the specification of 15 dB. A Firewall solution with one end grounded can be applied to improve the isolation (e.g., see U.S. patent application Ser. No. 10/453,798, Publication No. 20050041624 A1: “Systems and Methods that Employ a Dualband IFA-Loop CDMA Antenna and a GPS Antenna with a Device for Mobile Communication”, by P. Hui, J. V. Wonterghem and S. Eggleston). However, due to compactness of the phone, it is not feasible to introduce additional ground contact pin from the antenna carrier to the PWB (print wiring board) for this project.
Prior art solutions, including placing the BLUETOOTH/WLAN antenna away enough from the main antenna, or using grounded parasitic elements, are difficult to apply because of miniaturization requirements, therefore, an alternative solution is desired.